Insulated gate bipolar transistors (IGBTs) are an extremely attractive semiconductor device for power applications. They are more attractive than a power-type insulated gate field effect transistor (IGFET), which is popularly referred to as a MOSFET. An IGBT can handle both high voltages and high currents with small die size and with relatively low "on" resistance. In addition, an IGBT can be switched rapidly, making IGBTs potentially useful as switches in a three phase inverter for a high power alternating current motor application.
On the other hand, the high current density capability and low "on" resistance of the IGBT also present challenges. The possibility of device failure is aggravated when the IGBT is handling high power. The expression high power means current densities above about 135 amps per square centimeter of active chip area, at hundreds of volts. The expression high frequency switching means on/off frequencies above about 18 kilohertz, as for example 30 kilohertz. As might be expected, significant impedance, material and mechanical problems are encountered in handling such power at high frequencies and low resistances. This is especially true for a high power/high frequency module, in which several such IGBTs are electrically paralleled. Prior to the inventions of the related patents mentioned above, the foregoing problems were so difficult that not many high power/high frequency IGBT modules had been commercially manufactured. Of those that were made, they were made in low volume and each was individually specially crafted.
The related patents mentioned above describe a distinctive semiconductor substrate subassembly containing a high power/high frequency semiconductor switching device, and power modules containing the same. The subassemblies and modules are designed for manufacture on a commercial production basis. By commercial production basis, is meant production volumes such used in the automotive industry. The semiconductor subassemblies, and methods involving them, are particularly claimed in U.S. Pat. Nos. 5,492,842 and 5,539,254 both of whom were issued to Eytcheson et al. These and the other above-mentioned patents describe principles by which high quality and high performance subassemblies and modules can be more effectively manufactured on a commercial production basis. The above-mentioned patents describe semiconductor substrate subassemblies in which the upper surface of each semiconductor device has a plurality of filamentary wires bonded to it. In such a construction, the upper surface of the switching device has a plurality of contact pads and a plurality of filamentary wires bonded to the pads. More specifically, each contact pad on the semiconductor device has an end of at least one filamentary wire precisely located on it and bonded to it. The opposite end of each filamentary wire is bonded to an adjacent contact area on a supporting substrate for the semiconductor device.
In the subject invention, the plurality of filamentary wires extending from each device is replaced by a single terminal strap that is soldered in place. The terminal strap is unique in that it is partially metal and partially of composite material, both parts are solderable, the composite material part of the strap is soldered to the semiconductor chip contact pads, and the composite material part of the strap has a coefficient of thermal expansion close to the coefficient of thermal expansion of silicon. This unique terminal strap provides an electrical connection to the semiconductor switching device that is more durable during operation than the filamentary wires. However, it provides other benefits as well. For example, it provides a more mechanically durable subassembly during handling prior to assembly into a high power/high frequency module. A shorter emitter lead is provided, and it has improved parallelism and uniformity with the collector lead of the switching device. It would thus appear that subassembly parasitic electrical effects are reduced.
Use of these improved substrate subassemblies and methods, permit more economic manufacture of higher durability insulated gate bipolar transistor (IGBT) modules at automotive volumes. It is also believed that substrate subassemblies of lower electrical impedance are also obtained.